The invention provides an in-situ detection system for the concentration of methane in deep sea. The in-situ detection system is characterized in that by utilizing the characteristic that methane gas absorbs infrared light, methane components in a seawater water sample are separated by a gas-liquid separation technology, and then the content of the methane components is detected by using an infrared sensor. The detection system is composed of a flange disc, a decompression valve, a decompression cabin, a water inlet pump, a high-pressure-resisting water outlet pump, a gas pump, a solenoid valve, a circuit board, a gas-liquid separation optical detection chamber, a fixed bottom plate, a fixed bracket and a shell; and the appearance of the in-situ detection system is a small-size sealing device with a water inlet/outlet and an electric watertight connector. The in-situ detection system has a small response degree of non-methane gas and a high system signal-to-noise ratio; under a condition that ocean current of the deep sea fluctuates violently, the in-situ detection system has a strong anti-interference capability, a simple structure, a small size and low cost, and the integration of a small-size ocean monitoring system with relatively strong environment adaptability is convenient to realize; and the in-situ detection system can be distributed in a bad environment of the deep sea, and a concentration index of the methane in seawater can be subjected to in-situ detection under a long-time unmanned duty condition.